There is likely some dirt or sediment in the fill valve. Someone just posted a link to a nice piece by Toto on how to "service", i.e. clean, the valve to eliminate this. (Whenever I am away from my apt for a couple of weeks, I am amazed at how much sediment and junk comes out of the pipe when I open a faucet for the first time; some of that may have gotten caught in the valve.) Here's the official Toto piece: Link To Toto Fill Valve Service Instructions If you have the type B (Korky/Lavelle Made In USA) Valve, here's a video by Korky on how to service it (similar to the Toto instructions, but the video helps):

Usually just cleaning is good enough. If you need to replace the diaphragm cap on the Korky valve (Part R528), you can get it at Lowe's or HD for about $3 (or online or at many ACE's).

Let us know how it turns out. Usually just getting the gunk out and reassembling solves the problem.